Atactic polystyrenes with molecular weights from 460 to 1550000 were studied by calorimetric measurement.It was found that the samples having molecular weight higher than 103 produce initial DSC heating traces with well defined endothermic peaks in the vicinity of T g

the peak size decreased with the increase of molecular weight and then disappeared on rescanning.However

for the samples with molecular weights less than 103 no endotherm was observed in the same measurement conditions.The endothermic peaks in DSC traces suggest the existence of localised regions of segmental order in the samples and its possible origin must be the contribution of rr sequence in chains

because these samples have the different steric structures.13C-NMR determination indicates the proportion of rr triads is lower than 20% for samples of Mn3 and larger than 30% for those of Mn>103.Atactic polystyrenes used in the study were synthesised by solution polymerisation with nbutyllithium as initiator

benzene as solvent and alcohol as precipitant .The polymer concentrations were about 1～2% at the end of polymerisation.